In the field of industrial radiography, lead-foil screens are commonly used to generate secondary electron (photoelectron) emissions for intensification of the primary X-ray photon image. The photo electrons ejected from these screens do not interact with the test specimen. Emergent electrons, which act to increase the film response to changes in the intensity of the primary X-rays transmitted by the test specimen, are created by the interactions of the primary X-ray beam in the foil after having first traveled through the test specimen. The high specific ionization potential of electrons (about 100 times that of equivalent energy photons) increases the speed of radiography relative to a direct X-ray exposure without screens. Lead foil screens are also employed to reduce the influence of scatter, by absorbing secondary photons as well as electrons generated in the test specimen by the primary X-ray beam.
X-ray fluorescence and X-ray backscatter techniques have also been employed to study various materials and features, including the pattern of certain contamination defects and elemental compositions. In these processes, an X-ray signal is the primary objective of the analysis. In applications requiring film capture, various absorbers have been employed in these teachings to reduce the influence of electrons (Compton recoil and/or photoelectrons emitted from the specimen), which reduce the sharpness or contrast of the characteristic X-ray image. In the case of the subject invention, the X-rays 4 are deliberately hardened to desensitize the film 15F, 15B to the emergent X-ray beam 4, while recording the transmission of secondary electrons 9F, 9B passing through the test specimen 19F, 19B from an external target source 17.
Various means for producing an electron image have been reported in the prior art.
U.S. Pat. No. 3,758,778 dislcoses a technique in which low energy radiation, i.e., X-ray or infrared radiation, is used to trap electrons above the Fermi level at the surface of a test specimen. An electron microscope is used to accelerate and image the electrons to correlate to the surface features of the test specimen. The bombarding radiation source and secondary electron source are contained within the same vacuum envelope. In the present invention, on the other hand, an external medium-to-high-energy X- or gamma ray source 3 is used to eject a continuous spectrum of electrons 9F, 9B from the surface of a suitable target material 17 for imaging a detached test specimen 19F, 19B.
In the reference patent, the specimen temperature must be lowered below ambient in order to stabilize exothermic and/or exoelectron emission prior to creating a secondary surface emission by abruptly raising the specimen temperature above the Fermi level for exoelectron emission in response to the surface features, e.g., composition and topography, of the test specimen. In other words, the reference patent requires a controlled temperature environment. In the present invention, on the other hand, surface temperature is not a factor in creating an image of the uniformity and internal features of the test specimen 19F, 19B. The present invention does not require a controlled temperature environment.
In summary, the reference is a technique for recording surface features by stimulating electrons from within a test specimen; the electrons which contain the signal come from the specimen itself. On the other hand, the present invention records internal features at all points within the test specimen 19F, 19B using electrons 9F, 9B produced from a source 17 outside the test specimen 19F, 19B.
U.S. Pat. No. 2,967,240 discloses a method wherein surfaces of certain crystalline materials, e.g., piezoelectric monocrystals and dielectric crystals, are chemically etched in a caustic electrolyte while being simultaneously exposed to X-ray or electron beam radiation. An interface phenomenon acting to eject foreign ions from the electrolyte used for etching the crystals is observed spectroscopically. The pattern of interference is determined by the influence of flaws in the atomic crystalline structure, which flaws diffract the impinging radiation beam. Repeated or prolonged treatment may be used to reorder the crystalline lattice, thus eliminating the surface flaws. In the present invention, on the other hand, an etchant or external medium to sensitize or otherwise prepare the surface of the test specimen 19F, 19B for testing is not required or used. The present invention is not limited to recording an image of the crystalline state of the test specimen 19F, 19B.
In summary, the reference is a method which is sensitive to defects on or immediately below the surface of the test specimen. It employs spectroscopic analysis to reveal impurities in the specimen surface. On the other hand, the present invention is sensitive to defects throughout the test specimen 19F, 19B; internal impurities are revealed by direct imaging on a photographic plate 15F, 15B. The reference method employs low energy radiation bombardment from a discrete energy 50 Kev to 60 Kev source. The present invention employs medium-to-high-energy radiation bombardment 4, 7, and is not limited to a discrete energy photon source 3.
U.S. Pat. No. 2,939,012 describes nondestructive means whereby a beam of high energy electrons of known energy distribution is directed at the surface of an unknown test specimen. A photomultiplier crystal detector is used to measure the number of secondary electrons scattered at discrete energy levels as a function of the elemental composition of the specimen. The present invention, on the other hand, employs a capture film 15F, 15B which responds to electron particles which are transmitted (not absorbed and not widely scattered) by the specimen 19F, 19B, rather than scattered radiation; and the energy profile of the photon source 3 used to produce the electrons does not have to be known.
The reference employs a crystal detector to record backward scatter response; employs electron beam bombardment as a primary incident radiation source; and does not produce an image. The present invention uses a photographic film detector 15F, 15B to record forward or back emission electron 9F, 9B transmission response; and employs an X-ray or gamma ray source 3 as a primary particle stimulation source.
Secondary references are U.S. Pat. Nos. 2,382,739; 2,417,110; 4,316,087; and 4,366,380.